1. Introduction La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) shows very high

1. Introduction La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) shows very high

A
R
C
H
I
V
E
S
O
F
M
E
T A
L
L
Volume 58
U
R
G
Y
A
N
D
M
A T
E
R
I
A
2013
L
S
Issue 4
DOI: 10.2478/amm-2013-0171
M. MOSIAŁEK∗ , M. TATKO∗ , M. DUDEK∗∗ , E. BIELAŃSKA∗ , G. MORDARSKI∗
COMPOSITE Ag-La0.6 Sr0.4 Co0.8 Fe0.2 O3−δ CATHODE MATERIAL FOR SOLID OXIDE FUEL CELLS, PREPARATION AND
CHARACTERISTIC
KOMPOZYTOWY MATERIAŁ KATODOWY Ag-La0.6 Sr0.4 Co0.8 Fe0.2 O3−δ DO STAŁOTLENKOWYCH OGNIW PALIWOWYCH,
OTRZYMYWANIE I CHARAKTERYSTYKA
Composite cathodes Ag-La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ were obtained by two different procedures. In the first procedure porous
La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ (LSCF) matrix was prepared by sintering the LSCF paste, the matrix was then saturated with AgNO3
solution and sintered again. Introduced silver crystalized in the form of 10 nm crystallites in the whole LSCF matrix. In the second procedure the paste of silver powder was deposited on the surface of electrolyte and dried. The layer of silver paste was then
covered by a layer of the LSCF paste and sintered at 850◦ C. The following cells were tested: H2 |Ni-Ce0.8 Gd0.2 O1.9 |Ce0.8 Sm0.2 O1.9
|LSCF|O2 , H2 |Ni-Ce0.8 Gd0.2 O1.9 |Ce0.8 Sm0.2 O1.9 |LSCF-Ag|O2 and H2 |Ni-Ce0.8 Gd0.2 O1.9 |Ce0.8 Sm0.2 O1.9 |Ag|LSCF|O2 . Introduction
of silver interlayer between cathode and electrolyte increased output power by 18-28% whereas introduction of metallic silver
into porous LSCF caused increase in power by 14-18%.
Keywords: intermediate temperature solid oxide fuel cell, Ag-LSCF composite cathode, silver interlayer, samaria doped
ceria
Kompozytowe katody Ag-La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ wykonano dwoma sposobami. W pierwszej metodzie otrzymano porowatą matrycę La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ (LSCF) poprzez wyprażenie pasty LSCF, następnie nasączono ją roztworem AgNO3
i ponownie wyprażono. Wprowadzone srebro wykrystalizowało w postaci 10 nm krystalitów w matrycy LSCF. W drugiej metodzie na powierzchni elektrolitu położono pastę srebrną a po wysuszeniu pastę LSCF i wyprażono w 850◦ C. Testowano następujące ogniwa H2 |Ni-Ce0.8 Gd0.2 O1.9 |Ce0.8 Sm0.2 O1.9 |LSCF|O2 , H2 |Ni-Ce0.8 Gd0.2 O1.9 |Ce0.8 Sm0.2 O1.9 |LSCF-Ag|O2
i H2 |Ni-Ce0.8 Gd0.2 O1.9 |Ce0.8 Sm0.2 O1.9 |Ag|LSCF|O2 . Wprowadzenie warstwy srebra pomiędzy katodę a elektrolit podniosło moc
ogniwa o 18-28% podczas gdy wprowadzenie srebra do porowatego LSCF spowodowało wzrost mocy o 14-18%.
1. Introduction
La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ (LSCF) shows very high catalytic activity in the oxygen reduction reaction (ORR) as well as
high surface and bulk ionic conductivities [1, 2]. Ionic conductivity of LSCF is one of the highest among perovskites [3, 4].
However according to Tai et al. [5] LSCF shows relatively high
thermal expansion coefficient (TEC) in the temperature range
100-600◦ C (15.3×10−6 K−1 ) which is higher than TEC of zirconia based electrolytes (about 10.0×10−6 K−1 ) and close to
TEC of ceria based electrolytes (about 12.5×10−6 K−1 ). Ceria
based electrolytes show better ionic conductivity in the intermediate temperature range (500-700◦ C) than yttria-stabilized
zirconia, most frequently used solid electrolyte. In the IT range
Ce0.8 Sm0.2 O1.9 (samaria doped ceria – SDC), Ce0.8 Gd0.2 O1.9
(gadolinia doped ceria – GDC) and ceria doped by two cations
are the most frequently used electrolyte materials [6-8]. Zirconia electrolytes react with cathode materials containing cobalt
which results in the appearance of insulating phase. No such
reactions were observed in the case of ceria electrolytes.
∗
∗∗
Electrochemical impedance spectroscopy (EIS) is a powerful method allowing the evaluation of the performance of
prepared cathodes. Both charge transfer resistances and the
diffusional impedance may be measured by EIS. EIS is also
used in the study of the mechanism of oxygen reduction reaction (ORR) although well practice is to confirm the conclusion drawn from EIS using another electrochemical method.
Adler et al. [2] proposed mathematical description of the ORR
mechanism on porous cathodes. The model (Adler Lane Steel
model) they described predicted the Gerisher impedance shape
of the electrode impedance spectra and confirm it in the case of
the LSCF cathode at 700◦ C. The other description of ORR on
LSCF cathode was presented by Wang et al. [9]. They divided
the whole ORR reaction into 6 consecutive steps and deduced
the dependency of resistances on the oxygen partial pressure
for each step. They presented the EIS spectra containing more
futures than Adler et al. [2]. Properties of LSCF cathodes
depend not only on strontium or cobalt contents but also on
the grain sizes, porosity, oxygen vacancy concentration and on
the sintering time and temperature. The last two parameters
JERZY HABER INSTITUTE OF CATALYSIS AND SURFACE CHEMISTRY PAS, 30–239 CRACOW, POLAND
AGH – UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF FUELS AND ENERGY, AL. A. MICKIEWICZA 30, 30-059 KRAKÓW, POLAND
1342
can change area specific resistance by up to two orders of
magnitude [4]. LSCF cathode material can be improved by
addition of some electrolyte material or noble metals: GDC
[10], GDC and palladium [10, 11], GDC and silver [12, 13],
SDC and silver [14], Ag, Cu, Pt [15] and silver [12, 15-19].
In our previous paper [20] describing preliminary results
we found that application of simple impregnation method allowed us to prepare the composite cathode Ag-LSCF with improved performance compared to LSCF monophasic material.
The comparative tests performed at 700◦ C with solid oxide
fuel cells Ag-LSCF|SDC|Ni–GDC and LSCF|SDC|Ni-SDC
showed the increase of current and power density about by
33% for the cell containing silver as compared to cell involving monophasic cathode material.
The present work describes in more detailed manner the
composite Ag-LSCF obtained by impregnation method as well
as preparation and characterization of sandwich type composite cathode composed of LSCF layer and silver interlayer
placed between LSCF cathode and the electrolyte. The characteristics of the composite Ag-LSCF material as components
of solid oxide fuel cell operating in lower temperature range
is presented and discussed.
2. Experimental
2.1. Materials preparation
LSCF was prepared from nitrate solution by Pechinni
method. The starting materials were: La2 O3 (99.9%, ACROS),
Sr(NO3 )2 , Fe(NO3 ).3 9H2 O (99%, POCH), Co(NO3 ).2 6H2 O
(99%, POCH), citric acid (99.5%, POCH) and ethylene glycol (99.9%, POCH). Lanthanum nitrate was prepared by
dissolving lanthanum oxide in nitrate acid (65%, POCH).
The reagents were mixed in distilled water at the proportions enabling preparation of solid solution with the formula
La0.6 Sr0.4 Co0.8 Fe0.2 O3−δ . Citric acid and ethylene glycol were
added to the respective nitrate solutions. Water from the solutions was then evaporated at 180◦ C for 12 h, then at 200◦ C
for next 12 h and lastly at 220◦ C for 12 h to remove gaseous
reactants. Obtained material was then calcined at 1100◦ C for
3 h and then rotary-vibratory milled with a zirconia grinding
media in dry ethanol.
The electrode was made from LSCF powder by screen
printing method for testing the performance of obtained electrode material. Electrodes were prepared on the SDC electrolyte using ethylocelulose (ethoxyl content 49%, 100 cps,
Sigma-Aldrich) as pore former and terpineol (Sigma-Aldrich)
then calcined at 1100◦ C for 3 hours. Two different procedures
were used to prepare composite materials LSCF-Ag. In the
first one silver was introduced into porous structure of LSCF
in the form of the solution containing silver nitrate (99.9%,
POCH), citric acid and ethylene glycol then the electrode material was dried and calcined at 800◦ C. The procedure was
similar to described in [20]. In the second procedure firstly
the silver interlayer was put on the surface of SDC electrolyte
and on the top of silver the LSCF cathode was deposited.
2.2. Solid oxide fuel cell preparation
The SDC sintered samples were tested as oxide membranes in a two-chamber solid oxide fuel cell. An anode (50
wt%. NiO-GDC) powder (supplied by Fuel Cell Materials,
USA) was mixed with terpineol and ethyl cellulose to form a
slurry, subsequently screen-printed on the SDC electrolyte as
an anode, then heated at 1200◦ C for 1 h. The LSCF powder
was ground in a rotary-vibratory mill in dry ethylene alcohol
using zirconia-grinding media. The ground LSCF powder was
mixed with terpineol and ethyl cellulose to form an ink. The
ink was screen-printed through a 100-mesh silk screen onto
the SDC surface and heated at 400◦ C for 2 h to remove organic binders then heated at 950◦ C for 12 h with a heating
and cooling rate of 2◦ /min. For the modification of the LSCF
cathode with silver, a method of the nitrate decomposition was
applied. 0.05 mol dm−3 AgNO3 solution was dropped using a
suction pipette with an accuracy of 0.01 ml and poured into
the porous LSCF cathode layer placed on the heated hotplate
to evaporate water, following by firing at 800◦ C for 6 h. In the
second procedure the Ag thin layer from commercial paste
(Electroscience, UK) was deposited on the surface of SDC
electrolyte. Then, after drying Ag paste, the LSCF paste was
pasted on the top of the metallic layer. The layered composite
cathode materials were fired at 900◦ C for 1 h.
To prepare the Ag modified LSCF cathode, the firing
temperature was limited by the melting temperature of Ag
(961◦ C) to prevent agglomeration of the Ag species, which
can block the transport path for gaseous oxygen. Thus, the
temperature was lower than commonly used sintering temperatures of screen-printed LSCF cathodes, typically between
1000 and 1200◦ C. The cathode layer adhered well to the electrolyte layer. The low temperature melting of Ag improves the
adhesion of the LSCF cathode, and the ceramic impeded grain
growth of Ag to some extent.
2.3. Methods
The phase composition of all powders and sintered samples was identified by X-ray diffraction (XRD) analysis basing
on the ICDD data base. XRD measurements were performed
using the Panalytical X’Pert Pro system with monochromatic
Cu Ka radiation. The microstructure observations were carried
out using scanning electron microscope (SEM) using the apparatus JEOL JSM-7500F with INCA PentaFetx3 EDS system.
2.4. Electrochemical Impedance Spectroscopy (EIS)
measurements
The three electrode setup was used. The silver wire of
the diameter 1 mm of the shape of a flat spiral was used as
a counter electrode. Platinum foils contacted electrodes were
used as current collectors. All three electrodes were joined
together by pressing with an alumina rod pressed in turns
by a spring located in the cold space. The alumina test fixture was described in our previous work [21]. The EIS measurements were performed using Gamry 300 series potentiostat/galvanostat/ZRA (Gamry Instruments, USA), in oxygen
and oxygen-argon atmospheres at the oxygen partial pressure
from 0.001 to 1 atm. and temperature 600 and 550◦ C. The
frequency range used in the EIS measurements was from 0.01
1343
to 300000 Hz. The amplitude of the sinusoidal voltage signal
was 5 mV. In the analysis of the impedance data the program Minuit [22] based on a non-linear least-square regression procedure, adjusted to treat impedance data was used to
fit the equation describing the assumed equivalent electrical
circuit (EEC) to the measured data. The quality of the fit was
characterized by the standard deviation s, calculated from the
formula:
v
u
u
2
n Modulus(Z
tP
i, measured −Zi, f itted )
s=
1
Modulus(Zi, measured )
(1)
n−1
where Zi means the impedance at the frequency number i and
n is the number of frequencies in the impedance spectrum.
2.5. Electrochemical tests of solid oxide fuel cells
The following button type solid oxide fuel cells were test-
three capacitive arcs. The electrolyte arc is not visible due to
the frequency limitation and it is represented by resistance
R0 in the electric equivalent circuit (EEC) presented in Fig.
2b. Each visible arc is represented by the pair of resistor and
constant phase element (CPE). All three capacitive arcs are
oxygen concentration dependent. The diameters of high and
middle frequency arcs: represented by (R1 , CPE 1 ) and (R2 ,
CPE 2 ) pairs weakly depend on oxygen concentration whereas
the low frequency arc (R3 , CPE 3 ) invisible in pure oxygen and
50% oxygen grows with the decrease of the oxygen concentration. These diameters of semicircles reflects the resistances R1 ,
R2 and R3 . The fitted values of R1 , R2 and R3 are proportional
to (PO2 )n where n are equal –0.0033, –0.0055 and –0.167
respectively. The coefficient n for R3 is close to –0.125, the
value expected by Wang et al. [9] for following reaction:
O− + e− → O2−
(2)
ed:
H2 |Ni-GDC|SDC|LSCF|O2
H2 |Ni-GDC|SDC|LSCF–Ag|O2
H2 |Ni-GDC|SDC|Ag|LSCF|O2
The apparatus and procedure of electrochemical tests
were described in our previous paper [20]. The current and
voltage (I-V), power and current (P-I) characteristics were
measured by cyclic voltamtery using electrochemical station
Autolab connected with 20A Booster.
proceeding at the electrode|electrolyte interface as the rate determining step. The middle frequency arc has a shape similar
to the Gerisher impedance and can be interpreted by Adler
Lane Steel model although we used the pair (R2 , CPE 2 ) instead of the Gerisher impedance element in EEC and we obtained standard deviations s between 0.18 and 0.72%.
3. Results and discussion
3.1. XRD
The XRD analysis showed that application of Pechinni
method allowed preparation of a monophasic LSCF cathode
material. Only the reflexes that may be ascribed to either Ag
or LSCF phases were observed in the diffraction patterns of
composite materials which confirms that no foreign phases
due to the reaction between the AgNO3 solution and LSCF
were formed (Fig. 1).
Fig. 2. a) Impedance spectra of the Ag-LSCF electrode on SDC electrolyte at 600◦ C in oxygen and different oxygen argon atmospheres,
the numbers near symbols denote the logarithm of frequency;
b) equivalent electrical circuit
3.3. SEM micrographs of composite cathode Ag-LSCF
Fig. 3 presents the SEM microphotographs of the
Ag-LSCF composite cathode. Silver crystals of the size up
to 5 µm are randomly distributed on the surface of the porous
LSCF material. In the larger magnification the small silver particles of the size 10 nm are visible. These particles decorate
the whole LSCF matrix.
3.4. Electrochemical tests of solid oxide fuel cells
Fig. 1. XRD test of the absence of reaction between LSCF and
AgNO3 solution. Grey reflexes originate from LSCF and the dark
grey ones from metallic silver
3.2. EIS experiments on LSCF-Ag cathode
The EIS spectra of LSCF-Ag electrodes recorded at
600◦ C are presented in Fig. 2a. The shape of the spectra reveal
Figs. 4a-c shows the family of curves (V-I) and (P-I) for
cells (1-3) in the temperature range 500-700◦ C. As can be seen
for all investigated cells the open circuit voltage was close to
1 V values calculated from Nernst equation. In all cases the
increase of current and power density was observed for solid
oxide fuel cell with composite cathode LSCF-Ag in comparison to cathode made from LSCF only. One of the reasons of
such behavior may be the improvement of ORR kinetic on the
cathodic side.
1344
Fig. 3. SEM micrographs of Ag-LSCF composite cathode: a) silver
crystals on the surface; b) small silver agglomerates
Direct comparison of performance of both button solid
oxide fuel cells involving composite cathode LSCF-Ag with
each other showed that the introduction of Ag as interlayer between solid oxide electrolyte SDC and cathode LSCF caused
the higher improvement.
4. Conclusions
The shape of the EIS spectra measured on Ag-LSCF
electrode indicate that ORR on that electrode may proceeds through at least three slow steps, the low frequency arc can be ascribed to oxidation O− to O2− ion at the
electrode|electrolyte interface. Ag-LSCF|SDC|Ni-GDC and
Ni|-GDCSDC|Ag|LSCF reveal greater power output than
Ni|SDC|LSCF cell in the 500-750◦ C temperature range by
14-18% and 18-28% respectively.
Acknowledgements
This work was financially supported by Polish Ministry of
Science and Higher Education (grant No 6166/B/T02/2010/38) and
the EU Human Capital Operation Program, Polish Project No.
POKL.04.0101-00-434/08-00.
Fig. 4. The family of P-I curves recorded in the temperature range
500-750◦ C with SDC as a solid electrolyte with: a) monophasic LSCF
cathode; b) composite Ag-LSCF cathode c) LSCF cathode with the
silver interlayer
REFERENCES
[1] B.C.H. S t e e l e, J.M. B a e, Solid State Ionics 106, 255
(1997).
[2] S.B. A d l e r, J.A. L a n e, B.C.H. S t e e l e, J. Electrochem.
Soc. 143, 3554 (1996).
1345
[3] V. D u s a s t r e, J.A. K i l n e r, Solid State Ionics 126, 163
(1999).
[4] E.V. T s i p i s, V.V. K h a r t o n, J. Solid State Electr. 12,
1367 (2008).
[5] L.W. T a i, M.M. N a s r a l l a h, H.U. A n d e r s o n, D.M.
S p a r l i n, S.R. S e h l i n, Solid State Ionics 76, 273 (1995).
[6] J.W. F e r g u s, J. Power Sources 189, 30 (2006).
[7] S. P i n o l, M. M o r a l e, F. E s p i r e l l, J. Power Sources
169, 2 (2007).
[8] M. D u d e k, A. R a p a c z - K m i t a, M. M r o c z k o w s k a, M. M o s i a ł e k, G. M o r d a r s k i, Electrochim. Acta
55, 4387 (2010).
[9] Y. W a n g, L. Z h a n g, F. C h e n, C. X i a, J. Hydrogen
Energy 37, 8582 (2012).
[10] S. W a n g, T. K a t o, S. N a g a t a, T. H o n d a, T.
K a n e k o, N. I w a s h i t a, M. D o k i y a, Solid State Ionics
146, 203 (2002).
[11] S. W a n g, T. K a t o, S. N a g a t a, T. K a n e k o, N.
I w a s h i t a, T. H o n d a, M. D o k i y a, Solid State Ionics
152-153, 477 (2002).
[12] Y. S a k i t o, A. H i r a n o, N. I m a n i s h i, Y. T a k e d a,
O. Y a m a m o t o, Y. L i u, J. Power Sources 182, 476 (2008).
Received: 20 September 2013.
[13] K. M u r a t a, A. H i r a n o, N. I m a n i s h i, Y. T a k e d a,
ECS Transactions, 25, 2413 (2009).
[14] J. Z h a n g, Y. J i, H. G a o, T. H e, J. L i u, J. Alloy. Compd.
395, 322 (2005).
[15] J.T. H u a n g, X.D. S h e n, C.L. C h o u, J. Power Sources
187, 348 (2009).
[16] S.P. S i m n e r, M.D. A n d e r s o n, J.E. C o l e m a n, J.W.
S t e v e n s o n, J. Power Sources 161, 115 (2006).
[17] S.P. S i m n e r, M.D. A n d e r s o n, J.W. T e m p l e t o n,
J.W. S t e v e n s o n, J. Power Sources 168, 236 (2007).
[18] Y. L i u, M. M o r i, Y. F u n a h a s h i, Y. F u j i s h i r o, A.
H i r a n o, Electrochem. Commun. 9, 1918 (2007).
[19] S.H. J u n, Y.R. U h m, R.H. S o n g, C.K. R h e e, Curr.
Appl. Phys. 11, S305 (2011).
[20] M. M o s i a ł e k, M. D u d e k, J. W o j e w o d a - B u d k a,
Arch. Metall. Mater. 58, 275 (2013).
[21] M. M o s i a ł e k, E. B i e l a ń s k a, R.P. S o c h a, M.
D u d e k, G. M o r d a r s k i, P. N o w a k, J. B a r b a s z,
A. R a p a c z - K m i t a, Solid State Ionics 225, 755 (2012).
[22] F. J a m e s, M. R o o s, Comput. Phys. Commun. 10, 343
(1975).